Method for detecting a zero-point error of a coriolis gyroscope and coriolis gyroscope using said method

ABSTRACT

In a method for determining the zero-point error of a Coriolis gyro, the resonator of the Coriolis gyro has a disturbance force applied to it such that a change in the stimulation oscillation of the resonator is brought about. A change in the read oscillation of the resonator, caused by a partial component of the disturbance force, is extracted from a read signal which represents the read oscillation of the resonator as a measure of the zero-point error.

BACKGROUND

1. Field of the Invention

The present invention relates to Coriolis gyros. More particularly, thisinvention pertains to a method for determining the zero-point error of aCoriolis gyro.

2. Description of the Prior Art

Coriolis gyros (also known as “vibration gyros”) are increasinglyemployed for navigation. Such devices include a mass system that iscaused to oscillate. Such oscillation is generally a superimposition ofa large number of individual oscillations. The individual oscillationsof the mass system are initially independent of one another and each maybe regarded in the abstract as a “resonator”. At least two resonatorsare required for operation of a vibration gyro. A first resonator isartificially stimulated to oscillate, with such oscillations referred tobelow as a “stimulation oscillation”. A second resonator is stimulatedto oscillate only when the vibration gyro is moved or rotated. That is,Coriolis forces occur which couple the first resonator to the secondresonator, draw energy from the stimulation oscillation of the firstresonator, and transfer the energy to the read oscillation of the secondresonator. The oscillation of the second resonator is referred to belowas the “read oscillation”. In order to determine movement (in particularrotation) of the Coriolis, the read oscillation is tapped off and acorresponding read signal (e.g. the tapped-off read oscillation signal)is analyzed to determine whether any changes occurred in the amplitudeof the read oscillation that measures rotation of the Coriolis gyro.Coriolis gyros may be in the form of either an open loop or a closedloop system. In a closed loop system, the amplitude of the readoscillation is continuously reset to a fixed value (preferably zero) bycontrol loops.

FIG. 2 is a schematic diagram of a closed loop Coriolis gyro 1. The gyro1 has a mass system 2 that can be caused to oscillate and is referred tobelow as a resonator 2 (in contrast to the “abstract” resonators,mentioned above, which represent individual oscillations of the “real”resonator). As already mentioned, the resonator 2 may be regarded as asystem composed of two “resonators” (a first resonator 3 and a secondresonator 4). Each of the first and second resonators 3, 4 is coupled toa force transmitter (not shown) and to a tapping-off system (not shown).Noise produced by the force transmitter and the tapping-off system isindicated schematically by noise 1 (reference symbol 5) and noise 2(reference symbol 6).

The Coriolis gyro 1 includes four control loops. A first control loop isemployed for controlling the stimulation oscillation (i.e. the frequencyof the first resonator 3) at a fixed frequency (resonant frequency). Thefirst control loop has a first demodulator 7, a first low-pass filter 8,a frequency regulator 9, a VCO (voltage controlled oscillator) 10 and afirst modulator 11. A second control loop controls the stimulationoscillation at a constant amplitude and includes a second demodulator12, a second low-pass filter 13 and an amplitude regulator 14.

Third and fourth control loops are used for resetting forces thatstimulate the read oscillation. The third control loop includes a thirddemodulator 15, a third low-pass filter 16, a quadrature regulator 17and a second modulator 18. The fourth control loop comprises a fourthdemodulator 19, a fourth low-pass filter 20, a rotation rate regulator21 and a third modulator 22.

The first resonator 3 is stimulated at its resonant frequency ω1. Theresultant stimulation oscillation is tapped off, demodulated in phase bymeans of the first demodulator 7, and a demodulated signal componentpassed to the first low-pass filter 8 that removes the sum frequencies.The tapped-off signal is referred to below as the tapped-off stimulationoscillation signal. An output from the first low-pass filter 8 issupplied to a frequency regulator 9 that controls the VCO 10 as afunction of the applied signal so that the in-phase componentessentially tends to zero. For this, the VCO 10 sends a signal to thefirst modulator 11, which controls a force transmitter so that astimulation force is applied to the first resonator 3. When the in-phasecomponent is zero, the first resonator 3 oscillates at its resonantfrequency ω1. It should be mentioned that all of the modulators anddemodulators are operated on the basis of resonant frequency ω1.

The tapped-off stimulation oscillation signal is also passed to thesecond control loop and demodulated by the second demodulator 12. Theoutput of the second demodulator 12 is passed through the secondlow-pass filter 13, whose output signal is, in turn, applied to theamplitude regulator 14. The amplitude regulator 14 controls the firstmodulator 11 as a function of such signal and of a nominal amplitudetransmitter 23 such that the first resonator 3 oscillates at a constantamplitude (i.e. the stimulation oscillation has constant amplitude).

As has already been mentioned, movement or rotation of the Coriolis gyro1 results in Coriolis forces (indicated by the FC·cos(ω1·t) in thedrawing) that couple the first resonator 3 to the second resonator 4,causing the second resonator 4 to oscillate. A resultant readoscillation at frequency ω2 is tapped off so that a correspondingtapped-off read oscillation signal (read signal) is supplied to both thethird and fourth control loops. In the third control loop, this signalis demodulated by means of the third demodulator 15, the sum frequenciesremoved by the third low-pass filter 16, and the low-pass-filteredsignal supplied to quadrature regulator 17 whose output is applied tothe third modulator 22 so that corresponding quadrature components ofthe read oscillation are reset. Analogously, the tapped-off readoscillation signal is demodulated in the fourth control loop by means ofa fourth demodulator 19. It then passes through a fourth low-pass filter20 and the filtered signal is applied to a rotation rate regulator 21.The output of the rotation rate regulator 21 is proportional to theinstantaneous rotation rate and is passed as the rotation ratemeasurement to a rotation rate output 24 and to the second modulator 18,which resets the corresponding rotation rate components of the readoscillation.

A Coriolis gyro 1 as described above can be operated in either adouble-resonant form or in a form in which it is not double-resonant.When the Coriolis gyro 1 is operated in a double-resonant form, thefrequency of ω2 of the read oscillation is approximately equal to thefrequency ω1 of the stimulation oscillation. In contrast, when it isoperated in a form in which it is not double-resonant, the frequency ω2of the read oscillation differs from the frequency ω1 of the stimulationoscillation. In the case of double-resonance, the output signal from thefourth low-pass filter 20 contains information about the rotation rate,while, when it is not operated in double-resonant form, the outputsignal from the third low-pass filter 16 contains the rotation rateinformation. A doubling switch 25 which selectively connects the outputsof the third and fourth low-pass filters 16, 20 to the rotation rateregulator 21 and to the quadrature regulator 17 is provided forswitching between the double-resonant and non-double resonant modes.

Due to inevitable manufacturing tolerances, it is not possible to avoidthe force transmitter system that stimulates the first resonator(stimulation oscillation) while also slightly stimulating the secondresonator (read oscillation). The tapped-off read oscillation signalthus includes a part due to Coriolis forces and a part (undesirably) dueto manufacturing tolerances. The undesirable part results in theCoriolis gyro having a zero-point error whose magnitude is not possibleto distinguish between the two parts when tapping off the tapped-offread oscillation signal.

SUMMARY AND OBJECTS OF THE INVENTION

It is therefore the object of the present invention to provide a methodfor determining the zero-point error due to manufacturing tolerances ina Coriolis gyro.

The present invention addresses the above object by providing, in afirst aspect, a method for determining the zero-point error of aCoriolis gyro. A disturbance force is applied to the resonator of theCoriolis gyro to bring about a change in the stimulation oscillation ofthe resonator. A change in the read oscillation of the resonator,produced by a partial component of the disturbance force, is extracted,as a measure of zero-point error, from a read signal that represents theread oscillation of the resonator.

In a second aspect, the invention provides a Coriolis gyro. The gyro ischaracterized by a device that includes a disturbance unit that appliesa disturbance force to the resonator of the Coriolis gyro to modulatethe stimulation oscillation of the resonator. A disturbance signaldetection unit determines a disturbance component, produced by a partialcomponent of the disturbance force, contained in a read signal (whichrepresents the read oscillation) as a measure of the zero-point error.

The preceding and other features of the invention will become furtherapparent from the detailed description that follows. Such description isaccompanied by a set of drawings. Numerals of the drawings,corresponding to those of the written description, point to the featuresof the invention with like numerals referring to like featuresthroughout.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a Coriolis gyro based on the method ofthe invention;

FIG. 2 is a schematic diagram of a Coriolis gyro in accordance with theprior art;

FIG. 3 is a diagram for illustrating the interaction of a resonator, aforce transmitter system and a tapping-off system in a Coriolis gyro;

FIGS. 4A through 4D are a series of diagrams for illustrating the forcesand oscillation amplitudes of a Coriolis gyro with double resonance;

FIGS. 5A through 5D are a series of diagrams for illustrating the forcesand oscillation amplitudes of a Coriolis gyro near double resonance; and

FIGS. 6A through 6D are a series of diagrams for illustrating the methodof the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The general operation of a Coriolis gyro is explained below. In thisregard, reference will be made to FIGS. 3, 4A through 4D and 5A through5D.

FIG. 3 is a diagram for illustrating the interaction of a resonator, aforce transmitter system and a tapping-off system in a Coriolis gyro. Itschematically represents the Coriolis gyro as a system 40 comprising aresonator (not shown), a force transmitter system 41 and a tapping-offsystem 42. Possible oscillations x (stimulation) and y (read) areadditionally indicated that are coupled to one another by Coriolisforces resulting from rotations at right angles to the plane of thedrawing. The x oscillation (complex) is stimulated by the alternatingforce with complex amplitude Fx (in this case, only the real part Fxr).The y oscillation (complex) is reset by the alternating force at thecomplex amplitude Fy with the real part Fyr and the imaginary part Fyi.(The rotation vector exp(i*ω*t) is omitted in each case.)

FIGS. 4A through 4D are a series of diagrams for illustrating the forcesand oscillation amplitudes of a Coriolis gyro with double resonance.That is, they show the complex forces and complex oscillation amplitudesfor an ideal Coriolis gryo having the identical resonant frequency of xand y oscillations. The force Fxr and the stimulation frequency of thegyro are controlled to produce a purely imaginary, constant xoscillation. This is accomplished by means of an amplitude regulator 14that controls the magnitude, and a phase regulator 10 that controls thephase of the x oscillation. The operating ferquency ω1 is controlled sothat the x oscillation is purely imaginary (i.e., the real part of the xoscillation is zero.)

The Coriolis force during rotation, FC, is now purely real, since theCoriolis force is proportional to the speed of the x oscillation. Ifboth oscillations have the same resonant frequency, then the yoscillation, caused by the force FC, is as illustrated in FIG. 4D.

When double resonance is present, the real part of the taped-off ysignal is zero. It is not if double resonance is not present. In bothcases, the Coriolis force FC is zeroed. In the case of reset gyros, thisis accomplished by a regulator for Fyr, which compensates for FC. In thecase of Coriolis gyros operated with double resonance, the imaginarypart of y is zeroed by means of Fyr, and the real part of y is zeroed bymeans of Fyi. The bandwidth of the two control processes isapproximately 100 Hz.

The method of the invention will now be explained with reference to theschematic diagram of a resetting Coriolis gyro 1′ of FIG. 1. Theresetting Coriolis gyro 1′ additionally includes a disturbance unit 26,a demodulation unit 27, a regulator 28, a fifth low-pass filter 29 and amultiplier 30.

The disturbance unit 26 generates an alternating signal of frequencyωmod that is added to the output of the amplitude regulator. As analternative, band-limited noise can also be used as a disturbancesignal. Furthermore, this alternating signal is supplied to thedemodulation unit 27. The collated signal obtained in this way (outputfrom the amplitude regulator and alternating signal) is supplied to a(first) modulator 11 whose output is applied to a force transmitter (notshown), and, thus to the resonator 2. As a result, an alternating forcethat corresponds to the alternating signal is also applied to theresonator 2. Such alternating force can be observed, after “passingthrough” the resonator 2, in the form of a disturbance component on thetapped-off read oscillation signal.

In this example, the signal emitted from the rotation rate regulator issubjected to a demodulation process carried out by the demodulation unit27 at the frequency ωmod (disturbance frequency). The signal(disturbance component) obtained is filtered by the fifth low-passfilter 29 and supplied to the control unit 28. The signal supplied tothe control unit 28 represents a measure of the zero-point error. Thecontrol unit 28 produces an output signal as a function of the signalsupplied to it. Such output signal is supplied to the multiplier 30 andis in such a form that the disturbance component of the tapped-off readoscillation signal is controlled to be as small as possible. Themultiplier 30 multiplies the collated signal (output signal from theamplitude regulator and alternating signal) supplied to it by the outputfrom the control unit 28, and, thus, produces an output signal that isadded to the signal emitted from the rotation rate regulator. The biasof the Coriolis gyro is thus reset. The signal supplied to thedemodulation unit 27, which may also be the signal supplied to therotation rate regulator 21, or supplied to/emitted from the quadratureregulator 17. The signal supplied to the demodulation unit 27 may alsobe the tapped-off read oscillation signal itself. In the latter case,the operating frequency ω must also be accounted for during thedemodulation process.

In principle, it is possible to feed the output signal from themultiplier 30 into the rotation rate control loop at any desired point(not only directly upstream of the second modulator 18, i.e., at anydesired point between the point at which the read oscillation is tappedoff and the third modulator 22). Analogous considerations apply tofeeding the disturbance signal into the quadrature control loop.

Reference is now made to FIGS. 6A through 6D, a series of diagrams forfurther illustrating the method of the invention. The read oscillationwill in general “see” a small proportion of the stimulation force Fxr:kFyx*Fxr as a result of manufacturing tolerances. When the Fyr controlloop is closed, Fyr is thus changed by kFyx*Fyr when compared to thecorrect value. This results in a corresponding bias, as Fyr is a measureof the rotation rate.

To compensate for this error, the amplitude of Fxrt is modulated withoutany mean value by the disturbance unit 26. The modulation frequency (orfrequencies) of the band-limited modulation noise should be chosen sothat the stimulation oscillation is disturbed as little as possiblewhile the rotation rate control loop is disturbed as strongly aspossible (via the component kFyx*Kxr.) The error component in Fyr(kFyx*Fxr) is now compensated for by the addition of a controlledcomponent kFyxcomp*Fxr to Fyr in such a way that the modulation in therotation rate channel disappears. This is achieved by controllingkFyxcomp, which is emitted from the regulator unit 28 (preferably bysoftware). The input signal to a corresponding regulator (the regulatorunit 28) is the signal of Fyr, demodulated synchronously with themodulation frequency. When the regulator is matched, the modulationsignal in the rotation rate channel disappears, and there is no need fora blocking filter for the modulation frequency in the rotation rateoutput.

In this case, “resonator” refers to the entire mass system (or part ofit) that can be caused to oscillate in the Coriolis gyro (i.e., withreference to FIG. 2, that part of the Coriolis gyro that is annotatedwith reference numeral 2).

A major discovery on which the invention is based is that an artificialchange to the stimulation oscillation resulting from the application ofappropriate disturbance forces to the resonator can be observed in thetapped-off read oscillation signal: the change (modulation) of thestimulation oscillation also results in a change in the read oscillationdue to the manufacturing tolerances of the Coriolis gyro. That is, thedisturbance force is applied essentially to the first resonator, but apartial component of this disturbance force is also applied to thesecond resonator. The “penetration strength” of a disturbance such asthis to the tapped-off read oscillation signal is thus a measure of thezero-point error (“bias”) of the Coriolis gyro. If, therefore, thestrength of the disturbance component contained in the read signal isdetermined and compared with the strength of the disturbance force(change in the stimulation oscillation), the zero-point error can bederived. A disturbance component signal which is proportional to thedisturbance component can then be used to compensate directly for thezero-point error.

The disturbance forces are preferably produced by disturbance signalsthat are supplied to appropriate force transmitters, or are added tosignals which are supplied to the force transmitters. For example, adisturbance signal can be added to the respective control signals forcontrol of the stimulation oscillation, to produce the disturbanceforce.

The disturbance signal is preferably an alternating signal (e.g. asuperposition of sine-wave signals and cosine-wave signals). Analternating signal of this type produces an alternating force viacorresponding force transmitters that modulates the amplitude of thestimulation oscillation. The alternating signal is generally at a fixeddisturbance frequency so that the disturbance component of thetapped-off read oscillation signal can be determined by means of anappropriate demodulation process carried out at the disturbancefrequency.

The disturbance frequency of the disturbance signal/disturbance forcepreferably has a period which is substantially shorter than the timeconstant of the stimulation oscillation and of the same order ofmagnitude (or greater than) the time constant of the Coriolis gyro. Onealternative is to employ band-limited noise as a disturbance in theplace of an alternating signal. In such case, the disturbance componentis demodulated from the read signal by correlation of the noise signalwith the signal that contains the disturbance component, (e.g. thetapped-off read oscillation signal.)

The method described above can be used for both an open loop and aclosed loop Coriolis gyro. In the latter case, the zero-point error canbe compensated for as follows: a linear combination is formed of acontrolled part of an alternating signal, which produces the stimulationoscillation, preferably including the disturbance signal, and analternating signal which results in the read oscillation being reset.This is passed to a rotation rate control loop/quadrature control loopfor the Coriolis gyro. The controlled part is controlled so that thechange in the read oscillation (determined from the read signal) becomesas small as possible as a result of the modulation (i.e. the disturbancecomponent).

The disturbance component may, for example, be determined directly fromthe tapped-off read oscillation signal. The expression “read signal”covers this signal as well as the signal applied to a quadratureregulator in a quadrature control loop, or emitted from it, as well asthe signal applied to, or emitted from, a rotation rate regulator in arotation rate control loop.

If the disturbance force results from an alternating force at a specificdisturbance frequency, the disturbance signal detection unit has ademodulation unit by means of which the read signal is subjected to ademodulation process (a synchronous demodulation at the disturbancefrequency). This results in the disturbance component being determinedfrom the read signal. Alternatively, band-limited noise may be used asthe disturbance signal.

The Coriolis gyro is preferably resetting (i.e. it has a rotation ratecontrol loop and a quadrature control loop.) In a resetting Coriolisgyro, a control unit is advantageously provided to compensate for thezero-point error. A control unit produces a linear combination of acontrolled part of an alternating signal that produces the stimulationoscillation (preferably including the disturbance signal) and analternating signal. This results in resetting of the read oscillationand passing the collated signal to the rotation rate controlloop/quadrature control loop of the Coriolis gyro. The linearcombination of signals is controlled by the control unit so that thedisturbance component of the read oscillation, as determined from theread signal, becomes as small as possible. The zero-point error of theCoriolis gyro is thus compensated.

The disturbance signal detection unit preferably determines thedisturbance component from a signal that is emitted from a rotation rateregulator in the rotation rate control loop. The control loop in thisexample adds the linear combination of signals to an output signal fromthe rotation rate regulator.

While the invention has been described with reference to itspresently-preferred embodiment, it is not limited thereto. Rather, theinvention is limited only insofar as it is defined by the following setof patent claims and includes within its scope all equivalents thereof.

1. A method for determining the zero-point error of a Coriolis gyro,wherein the resonator of the Coriolis gyro has a disturbance forceapplied to it such that a change in the stimulation oscillation of theresonator is brought about, and a change in the read oscillation of theresonator, which is produced by a partial component of the disturbanceforce, is extracted from a read signal which represents the readoscillation of the resonator as a measure of the zero-point error. 2.The method as claimed in claim 1, characterized in that the disturbanceforce is an alternating force which modulates the amplitude of thestimulation oscillation.
 3. The method as claimed in claim 2,characterized in that the disturbance force has a disturbance frequencywhose period is substantially shorter than the time constant of thestimulation oscillation but is of the same order of magnitude as orgreater than the time constant of the Coriolis gyro.
 4. The method asclaimed in claim 2, characterized in that the change in the readoscillation is detected by subjecting the read signal to a demodulationprocess on the basis of a disturbance frequency.
 5. The method asclaimed in claim 1, characterized in that the disturbance force isproduced by a disturbance signal which is band-limited noise.
 6. Themethod as claimed in claim 1, characterized in that a linear combinationis formed of a controlled part of an alternating signal, which producesthe stimulation oscillation, and an alternating signal, which results inthe read oscillation being reset, and is passed to a rotation ratecontrol loop/quadrature control loop for the Coriolis gyro, in such away that the change in the read oscillation determined from the readsignal becomes as small as possible.
 7. A Coriolis gyro, characterizedby a device for determining the zero-point error of the Coriolis gyro,having: a disturbance unit which applies a disturbance force to theresonator of the Coriolis gyro such that the stimulation oscillation ofthe resonator is modulated, a disturbance signal detection unit, whichdetermines a disturbance component which is contained in a read signal(which represents the read oscillation) and has been produced by apartial component of the disturbance force, as a measure of thezero-point error.
 8. The Coriolis gyro as claimed in claim 7,characterized by a rotation rate control loop/quadrature control loop.9. The Coriolis gyro as claimed in claim 8, characterized by a controlunit, which forms a linear combination of a controlled part of analternating signal, which produces the stimulation oscillation, and analternating signal which results in the read oscillation being reset,and passes it to the rotation rate control loop/quadrature control loopfor the Coriolis gyro, with the control unit controlling the linearcombination of the signals such that the disturbance component, which isdetermined from the read signal, of the read oscillation becomes assmall as possible.
 10. The Coriolis gyro as claimed in claim 9,characterized in that the disturbance signal detection unit determinesthe disturbance component from a signal which is emitted from a rotationrate regulator in the rotation rate control loop, and the linearcombination of the signals is added to an output signal from therotation rate regulator.